Two multi-level memory cells sensed to determine multiple data values

ABSTRACT

The present disclosure includes apparatuses, methods, and systems for sensing two memory cells to determine multiple data values. An embodiment includes a memory having a plurality of memory cells and circuitry configured to sense memory states of each of two self-selecting multi-level memory cells (MLC) of the plurality of memory cells to determine multiple data values. The data values are determined by sensing a memory state of a first MLC using a first sensing voltage in a sense window between a first threshold voltage distribution corresponding to a first memory state and a second threshold voltage distribution corresponding to a second memory state and sensing a memory state of a second MLC using a second sensing voltage in a sense window between the first threshold voltage distribution corresponding to a first memory state and a second threshold voltage distribution corresponding to the second memory state. The sequence of determining data values includes sensing the memory state of the first and the second MLCs using higher sensing voltages than the first and the second sensing voltages in subsequent sensing windows, in repeated iterations, until the state of the first and the second MLCs are determined. The first and second sensing voltages are selectably closer in the sense window to the first threshold voltage distribution or the second threshold voltage distribution.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory andmethods, and more particularly, to two multi-level memory cells sensedto determine multiple data values.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits and/or external removable devices in computers orother electronic devices. There are many different types of memory,including volatile and non-volatile memory. Volatile memory can requirepower to maintain its data and can include random-access memory (RAM),dynamic random access memory (DRAM), and synchronous dynamic randomaccess memory (SDRAM), among others. Non-volatile memory can providepersistent data by retaining stored data when not powered and caninclude NAND flash memory, NOR flash memory, read only memory (ROM), andresistance variable memory such as phase change random access memory(PCRAM), resistive random access memory (RRAM), magnetic random accessmemory (MRAM), and programmable conductive memory, among others.

Memory devices can be utilized as volatile and non-volatile memory for awide range of electronic applications in need of high memory densities,high reliability, and/or low power consumption. Non-volatile memory maybe used in, for example, personal computers, portable memory sticks,solid state drives (SSDs), digital cameras, cellular telephones,portable music players, such as MP3 players, and movie players, amongother electronic devices.

Resistance variable memory devices can include resistance variablememory cells that can store data based on the resistance state of astorage element (e.g., a memory element having a variable resistance).As such, resistance variable memory cells can be programmed to storedata corresponding to a target memory state by varying the resistancelevel of the memory element. Resistance variable memory cells can beprogrammed to a target memory state (e.g., corresponding to a particularresistance state) by applying sources of an electrical field or energy,such as positive or negative electrical pulses (e.g., positive ornegative voltage or current pulses) to the memory cells (e.g., to thememory element of the memory cells) for a particular duration. A stateof a resistance variable memory cell can be determined by sensingcurrent through the memory cell responsive to an applied interrogationvoltage. The sensed current, which varies based on the resistance levelof the memory cell, can indicate the state of the memory cell.

Various memory arrays can be organized in a cross-point architecturewith memory cells (e.g., resistance variable memory cells) being locatedat intersections of a first and second signal lines used to access thememory cells (e.g., at intersections of word lines and bit lines). Someresistance variable memory cells can comprise a select element (e.g., adiode, transistor, or other switching device) in series with a storageelement (e.g., a phase change material, metal oxide material, and/orsome other material programmable to different resistance levels). Someresistance variable memory cells, which may be referred to asself-selecting memory cells, can include a single material which canserve as both a select element and a storage element for the memorycell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three-dimensional view of an example of a memory array, inaccordance with an embodiment of the present disclosure.

FIG. 2A is an example of a current-versus-voltage curve, in accordancewith an embodiment of the present disclosure.

FIG. 2B is another example of sensing threshold voltage distributionsassociated with memory states of memory cells for two multi-level memorycells being sensed to determine multiple data values, in accordance withanother embodiment of the present disclosure.

FIG. 3A illustrates an example of sensing threshold voltagedistributions associated with memory states of memory cells for twomemory cells being sensed to determine multiple data values, inaccordance with an embodiment of the present disclosure.

FIG. 3B illustrates an example of a truth table associated with memorystates of memory cells for two memory cells being sensed to determinemultiple data values, in accordance with an embodiment of the presentdisclosure.

FIG. 4 is a block diagram illustration of an example apparatus, inaccordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure includes apparatuses, methods, and systems forsensing two memory cells to determine multiple data values. Anembodiment includes a memory having a plurality of memory cells andcircuitry configured to sense memory states of each of twoself-selecting multi-level memory cells (MLC) of the plurality of memorycells to determine multiple data values. The data values are determinedby sensing the memory state of a first sensing a memory state of a firstMLC using a first sensing voltage in a sense window between a firstthreshold voltage distribution corresponding to a first memory state anda second threshold voltage distribution corresponding to a second memorystate and sensing a memory state of a second MLC using a second sensingvoltage in a sense window between the first threshold voltagedistribution corresponding to a first memory state and a secondthreshold voltage distribution corresponding to the second memory state.The sequence of determining data values includes sensing the memorystate of the first and the second MLCs using higher sensing voltagesthan the first and the second sensing voltages in subsequent sensingwindows, in repeated iterations, until the state of the first and thesecond MLCs are determined. The first sensing voltage and the secondsensing voltage are selectably closer in the sense window to the firstthreshold voltage distribution or the second threshold voltagedistribution.

Embodiments of the present disclosure can provide benefits, such asincreased reliability and accuracy of data sensing and/or increasedspeed and reduced complexity of data correction operations, as comparedto previous memory devices. For example, previous approaches forprogramming resistance variable memory cells, such as self-selectingmemory cells, may generate one of two different states, such that eachof the memory cells can be programmed with multiple data values (e.g.,00, 01, 10 or 11). In such approaches, two sensing voltages may beplaced substantially at a median voltage separation in a sense (e.g.,read) window between a first threshold voltage distributioncorresponding to a first memory state with multiple data values and asecond threshold voltage distribution (e.g., a distribution of storedvoltages for a second number of memory cells) corresponding to a secondmemory state with multiple data values. Each of the threshold voltagedistributions may represent separate distributions of stored voltagesfor a plurality of memory cells and/or a statistical distribution ofpotential stored voltages for a first memory cell and a paired secondmemory cell.

A number of data storage and/or programming considerations for varioustypes of memory cells (e.g., resistance variable memory cells, amongothers), however, may cause stored voltages of a number (e.g., a subsetor all) of the memory cells in the threshold voltage distributions toshift and to overlap the single sensing voltage at the median of thesense window such that the sensed voltage relative to the sensingvoltage (e.g., the data value) becomes unreliable and/or inaccurateduring read operations. Such shifts may, for example, be contributed toby (e.g., result from) at least one of a widening (e.g., over time)threshold voltage distributions to at least overlap the single sensingvoltage, a drift (e.g., over time) of threshold voltage distributions toa larger median voltage to at least overlap the single sensing voltage,and/or a disturbance of memory states of subsets of memory cells in thethreshold voltage distributions, by performance of read and/or writeoperations on at least some of the memory cells, to at least overlap thesingle sensing voltage, among other possible data storage and/orprogramming considerations.

The embodiments described herein for sensing two memory cells todetermine multiple data values by using multiple sensing voltages thatare selectably closer in the sense window to various threshold voltagedistributions are intended to reduce a potential for some of the storedvoltages overlapping the sensing voltages of the respective thresholdvoltage distributions. As such, these embodiments can increase thereliability and accuracy of data accessed from memory cells. Moreover,as described herein, the embodiments can increase speed and/or reducecomplexity of data correction operations for potential data errors thatmay result, for example, from such shifts in the first and/or secondthreshold voltage distributions to further increase the reliability andaccuracy of the data accessed from memory cells, as compared to previousmemory devices. Memory devices may include a plurality of arrays ofmemory cells of which all of the arrays or a subset (less than all) ofthe arrays may be configured to store, read, write, and/or perform datavalidation and correction as described herein.

As used herein, “a”, “an”, or “a number of” can refer to one or more ofsomething, and “a plurality of” can refer to two or more such things.For example, a memory device can refer to one or more memory devices,and a plurality of memory devices can refer to two or more memorydevices. Additionally, the designators “N” and “M”, as used herein,particularly with respect to reference numerals in the drawings,indicates that a number of the particular feature so designated can beincluded with a number of embodiments of the present disclosure.Moreover, numbers separated by a hyphen from a particular referencenumeral (e.g., 312-1, 312-2, 312-3 in FIG. 3A for sense windows) areintended to denote similar features, although the position, magnitude,width, height, and/or shape, etc., of such features may vary withinand/or between particular embodiments. The figures herein follow anumbering convention in which the first digit or digits correspond tothe drawing figure number and the remaining digits identify an elementor component in the drawing. Similar elements or components betweendifferent figures may be identified by the use of similar digits.

FIG. 1 is a three-dimensional view of an example of a memory array 100(e.g., a cross-point memory array), in accordance with an embodiment ofthe present disclosure. Memory array 100 may include a plurality offirst signal lines (e.g., first access lines), which may be referred toas word lines 110-0 to 110-N, and a plurality second signal lines (e.g.,second access lines), which may be referred to as bit lines 120-0 to120-M that cross each other (e.g., intersect in different planes). Forexample, each of word lines 110-0 to 110-N may cross bit lines 120-0 to120-M. A memory cell 125 may be between the bit line and the word line(e.g., at each bit line/word line crossing).

The memory cells 125 may be resistance variable memory cells, forexample. The memory cells 125 may include a material programmable todifferent memory states. In some examples, each of memory cells 125 mayinclude a single material that may serve as a select element (e.g., aswitching material) and a storage element, so that each memory cell 125may act as both a selector device and a memory element. Such a memorycell may be referred to herein as a self-selecting memory cell. Forexample, each memory cell may include a chalcogenide material that maybe formed from various doped or undoped materials, that may or may notbe a phase-change material, and/or that may or may not undergo a phasechange during reading and/or writing the memory cell. In some examples,each memory cell 125 may include a ternary composition that may includeselenium (Se), arsenic (As), and germanium (Ge), a quaternarycomposition that may include silicon (Si), Se, As, and Ge, etc.

In various embodiments, the threshold voltages of memory cells 125 maysnap back in response to a magnitude of an applied voltage differentialacross them exceeding their threshold voltages. Such memory cells may bereferred to as snapback memory cells. For example, a memory cell 125 maychange (e.g., snap back) from a non-conductive (e.g., high impedance)state to a conductive (e.g., lower impedance) state in response to theapplied voltage differential exceeding the threshold voltage. Forexample, a memory cell snapping back may refer to the memory celltransitioning from a high impedance state to a lower impedance stateresponsive to a voltage differential applied across the memory cellbeing greater than the threshold voltage of the memory cell. A thresholdvoltage of a memory cell snapping back may be referred to as a snapbackevent, for example.

The example shown in FIG. 1 may include a driver (e.g., a word linedriver—not shown) coupled to word line 110. The word line driver maysupply bi-polar (e.g., positive and negative) current and/or voltagesignals to word line 110. A sense amplifier (e.g., in the sensingcircuitry 405 shown in FIG. 4), which may comprise a cross-coupledlatch, may be coupled to the word line driver and may detect positiveand negative currents and/or positive and negative voltages on word line110. In some examples, a sense amplifier may be part of (e.g., includedin) a word line driver. For example, the word line driver may includethe sensing functionality of sense amplifier. In some examples, a senseamplifier may be part of (e.g., included in) the sensing circuitrycoupled to the memory cells 125 (e.g., in an array of memory cells, asshown at 406 in FIG. 4). A bit line driver (not shown) may be coupled tobit line 120 to supply positive and/or negative current and/or voltagesignals to bit line 120.

The sense amplifier may detect the current and/or voltage associatedwith one or more memory cells 125 relative to a particular sensingvoltage (e.g., threshold) and may output a signal to indicate aparticular memory state of each memory cell 125 based on a magnitude ofa voltage stored by a particular memory cells relative to the particularsensing voltage. The sensed memory state for each memory cell 125 may besent to and/or stored by a particular latch (not shown) as expressed ina truth table (e.g., as shown at 313 and described in connection withFIG. 3B), and the sensed memory states value may represent a particulardetermined outcome of verification of validity of the sensed memorystates and/or the need for correction of invalid sensed memory states(e.g., associated with a read operation).

FIGS. 2A and 2B illustrate memory cell snapback as described herein.VCELL can represent an applied voltage across the memory cell. Forexample, VCELL can be a voltage applied to a top electrode correspondingto the memory cell minus a voltage applied to a bottom electrodecorresponding to the memory cell (e.g., via a respective word line andbit line). As shown in FIG. 2A, responsive to an applied positivepolarity voltage (VCELL), a memory cell programmed to memory state 1(e.g., in a threshold voltage distribution) is in a non-conductive stateuntil VCELL reaches voltage Vtst02, at which point the memory celltransitions to a conductive (e.g., lower resistance) state. Thistransition can be referred to as a snapback event, which occurs when thevoltage applied across the memory cell (in a particular polarity)exceeds the memory cell's threshold voltage. Accordingly, voltage Vtst02can be referred to as a snapback voltage. In FIG. 2A, voltage Vtst01corresponds to a snapback voltage for a memory cell programmed to memorystate 1 (e.g., in another threshold voltage distribution). That is, asshown in FIG. 2A, the memory cell transitions (e.g., switches) to aconductive state when VCELL exceeds Vtst01 in the negative polaritydirection.

Similarly, as shown in FIG. 2B, responsive to an applied negativepolarity voltage (VCELL), a memory cell programmed to memory state 0(e.g., in a threshold voltage distribution) is in a non-conductive stateuntil VCELL reaches voltage Vtst11, at which point the memory cell snapsback to a conductive (e.g., lower resistance) state. In FIG. 2B, voltageVtst12 corresponds to the snapback voltage for a memory cell programmedto memory state 0 (e.g., in another threshold voltage distribution).That is, the memory cell snaps back from a high impedance non-conductivestate to a lower impedance conductive state when VCELL exceeds Vtst12 inthe positive polarity direction.

In various instances, a snapback event can result in a memory cellswitching memory states. For instance, if a VCELL exceeding Vtst02 isapplied to a memory cell in memory state 1, the resulting snapback eventmay reduce the threshold voltage of the memory cell to a level below arespective sensing voltage, as described herein, which would result inthe memory cell being sensed (read) as memory state 0 rather than memorystate 1. As such, in a number of embodiments, a snapback event canswitch a memory cell to the opposite state (e.g., from memory state 1 tomemory state 0 and vice versa), as described in connection with FIG. 3A.

FIG. 3A illustrates an example of sensing threshold voltagedistributions associated with memory states of memory cells for twomulti-level memory cells (MLCs) being sensed to determine multiple datavalues, in accordance with an embodiment of the present disclosure.

The example shown in FIG. 3A illustrates four threshold voltagedistributions 308-1, 308-2, 308-3, and 308-4, representing four bits,associated with the memory states of a first one of a pair of MLCs,e.g., a first MLC memory cell 301-1, and four threshold voltagedistributions 309-1, 309-2, 309-3, and 309-4, associated with the memorystates of a second one of the pair of MLCs, e.g., a second MLC memorycell 301-2, in a negative polarity orientation. The memory cell 301-1shown comprise 2 bits. Each bit has 2 values and the combinations ofdata values may produce 4 memory states. The second memory cell 301-2may also comprise 2 bits. Two memory cells, 301-1 and 301-2respectively, that may operate as a pair of memory cells to each store avoltage magnitude that, when sensed and compared, enable determinationof multiple data values. Memory cell 301-1 represents one of a pair ofsuch memory cells, where each of the memory cells is configured toselectably store a voltage magnitude (e.g., relative to zero volts (0V))corresponding to four memory states as determined by reference to adetermined (e.g., predetermined) sensing voltage in a sense window. Inthe example embodiment of FIG. 3A, three (3) sense windows 312-1, 312-2,and 312-3 are shown between four memory states. For example, sensewindow 312-1 is shown between threshold voltage distribution 308-1 (datavalue 11) and threshold voltage distribution 308-2 (data value 10).Sense window 312-2 is shown between threshold voltage distribution 308-2(data value 10) and threshold voltage distribution 308-3 (data value01). Sense window 312-3 is shown between threshold voltage distribution308-3 (data value 01) and threshold voltage distribution 308-4 (datavalue 00). When the threshold voltage distributions have not shiftedrelative to (e.g., are not overlapping) one or both of their determinedsensing voltages, each corresponds to two of the four data values.

In the example of FIG. 3A, six (6) sensing voltages are shown associatedwith the three sense windows, two for each sense window. For example,sensing voltages 310-1 (D*VDM1N) and 311-1 (D VDM1N) are associated withsense window 312-1. Sensing voltages 310-2 and 311-2 are associated withsense window 312-2. Sensing voltages 310-3 and 311-3 are associated withsense window 312-3. The memory cells may be determined to have switchedmemory states due to a shift in a threshold voltage distributionrelative to a magnitude of the sensing voltage closest to the thresholdvoltage distribution in the sense window. For example, memory cell 301-1may be determined to have switched memory states due to a shift in thefirst threshold voltage distribution 308-1 relative to a magnitude ofthe first sensing voltage 310-1 (D*VDM1N) in the sense window 312-1.

The first memory cell 301-1 may be programmed to a data valuerepresented by the threshold voltage distribution 308-1 having a datavalue state (11) while the second memory cell 301-2 may be programmed toa data value represented by the threshold voltage distribution 309-2having a data value (10). In order to read the data values of the firstmemory cell 301-1 a first sensing voltage 310-1 may be applied in asense window 312-1 between a first threshold voltage distribution 308-1corresponding to a first data value (11) and a second threshold voltagedistribution 308-2 corresponding to a second data value (10). To readthe memory states of the second memory cell 301-2, a second sensingvoltage 311-1 (D VDM1N) may be applied in a sense window 312-1 between afirst threshold voltage distribution 309-1 corresponding to a first datavalue (11) of the second memory cell 301-2 and a second thresholdvoltage distribution 309-2 corresponding to a second data value (10) ofthe second memory cell 301-2.

In this embodiment, the first sensing voltage 310-1 (D*VDM1N) and thesecond sensing voltage 311-1 (D VDM1N) are selectably closer (e.g.,while having a statistically appropriate separation margin) in the sensewindow 312-1 to the first threshold voltage distribution 308-1 or thesecond threshold voltage distribution 308-2 of the first memory cell301-1 and the first and second threshold voltage distribution, 309-1 and309-2, of the second memory cell 301-2. For example, as illustrated inFIG. 3A, first sensing voltage 310-1 (D*VDM1N) applied to the first andsecond memory cells, 301-1 and 301-2, is closer (e.g., while having astatistically appropriate separation margin) to first threshold voltagedistribution 308-1 or 309-1 in sense window 312-1, respectively. Thesecond sensing voltage 311-1 (D VDM1N) is closer (e.g., while having astatistically appropriate separation margin) to the second thresholdvoltage distribution 308-2 or 309-2 in sense window 312-1, respectively.A sensing voltage determines a reliability of a threshold voltagedistribution. In some embodiments, the memory cells may be read from thelower sensing voltage to the highest sensing voltage. That is, firstsensing voltage may have a lesser, absolute value, voltage magnitudethan the second sensing voltage in the negative or positive polarity.

To sense the memory state of the first memory cell 301-1 a third sensingvoltage 310-2 is applied in a sense window 312-2 between the secondthreshold voltage distribution 308-2 corresponding to the second memorystate (10) and a third threshold voltage distribution 308-3corresponding to a third memory state (01). A memory state of the secondmemory cell 301-2 may be sensed (e.g., read) by applying a fourthsensing voltage 311-2 in the sense window 312-2 between the secondthreshold voltage distribution 309-2 corresponding to the second memorystate (10) and a third threshold voltage distribution 309-3corresponding to the third memory state (01). In this embodiment, thethird sensing voltage 310-2 and the fourth sensing voltage 311-2 areselectably closer in the sense window 312-2 to the second thresholdvoltage distribution 308-2 or the third threshold voltage distribution309-3. For example as illustrated in FIG. 3A, third sensing voltage310-2 is closer to second threshold voltage distribution 308-2 in thefirst memory cell 301-1 in sense window 312-2 while fourth sensingvoltage 311-2 is closer to the third threshold voltage distribution309-3 in sense window 312-2 in the second memory cell 301-2.

Lastly, a fifth sensing voltage 310-3 may be applied to sense the memorystate of the first memory cell 301-1 in a sense window 312-3 between thethird threshold voltage distribution 308-3 corresponding to the thirdmemory state (01) and a fourth threshold voltage distribution 308-4corresponding to a fourth memory state (00). The memory state of thesecond memory cell 301-2 may be sensed using a sixth sensing voltage311-3 in the sense window 312-3 between the third threshold voltagedistribution 309-3 corresponding to the third memory state (01) and afourth threshold voltage distribution 309-4 corresponding to the fourthmemory state (00). In this embodiment, the fifth sensing voltage 310-3and the sixth sensing voltage 311-3 are selectably closer in the sensewindow 312-3 to the third threshold voltage distribution 308-3 or thefourth threshold voltage distribution 309-4, respectively. For exampleas illustrated in FIG. 3A, fifth sensing voltage 310-3 is closer tothird threshold voltage distribution 308-3 in the first memory cell301-1 in sense window 312-3 while sixth sensing voltage 311-3 is closerto the fourth threshold voltage distribution 309-4 in sense window 312-3in the second memory cell 301-2. Although, memory cell 301-1 and memorycell 301-2 are described and illustrated in detail in a negativepolarity, embodiments are not so limited. Memory cell 301-1 and memorycell 301-2 may be similarly read in a positive polarity.

The circuitry may be configured to determine data values (“states”), bycomparing the sensed memory states of each of the two multi-level memorycells, validating (e.g., determining “valid” states for a pair ofmultilevel cells (MLCs shown in FIG. 3B) a memory state of memory cell301-1 by determining that the sensed data value state is sensed asprogrammed, and determining a switch in a memory state of the two memorycells (e.g., either of memory cells 301-1 or 301-2) based at least inpart on one combination of the two memory states that are sensed (e.g.,read) being more readably reliable than another correspondingcombination of the two sensed memory states. The reliability may be dueto a magnitude of separation of at least one sensing voltage from onethreshold voltage distribution on one side of a sensing window, e.g.,312-1, 312-2, 312-3, being nearer or farther apart than a separation ofthe at least one sensing voltage from the other threshold voltagedistribution of the other memory state.

As presented in the example of FIG. 3A, the six (6) sensing voltageswith reference numerals 310 and 311 may be used to determine the memorystates of a corresponding pair of multilevel memory cells (e.g., todistinguish between memory states as part of a read operation).Determining the reliability and/or accuracy of the data values maydepend on an ability to distinguish, for example, data value state (10),corresponding to threshold voltage distribution 308-2, for first memorycell 301-1 from data value state (11), corresponding to thresholdvoltage distribution 308-1, for first memory cell 301-1 and also fromdata value state (10), corresponding to threshold voltage distribution309-2, for second memory cell 301-2 from data value state (11),corresponding to threshold voltage distribution 309-1, for second memorycell 301-2.

A determination that the one combination of sensed memory states is morereadably reliable may be based on the greater magnitude of theseparation, thereby having a corresponding greater shift to reach thesensing voltage being less probable than a shift resulting from a lesserseparation in the other combination of sensed memory states.Accordingly, as shown in FIG. 3A, sensing voltage 310-1 (D*VDM1N) isused for memory cell 301-1 and is at a greater separation from thresholdvoltage distribution 308-2, having a data value of 10, than theseparation of sensing voltage 310-1 (D*VDM1N) from threshold voltagedistribution 308-1, having a data value of 11. In addition, sensingvoltage 311-1 (D VDM1N) is used for memory cell 301-2 and is at agreater separation from threshold voltage distribution 309-1, having adata value of 11, than the separation of sensing voltage 311-1 (D VDM1N)is from threshold voltage distribution 309-2, having a data value of 10.

As used herein, being readably reliable is intended to mean the sensedmemory states being more reliable than, for example as shown in FIG. 3A,threshold voltage distribution 308-1, having a data value of 11, usingsensing voltage 310-1 (D*VDM1N) for memory cell 301-1 and sensing ofthreshold voltage distribution 309-2, having a data value of 10, usingsensing voltage 311-1 (D VDM1N) for the intended memory cell 301-2.

The present disclosure further describes the ability, in some instances,to determine which of two memory cells (e.g., memory cells 301-1 or301-2) has switched their memory state due to a shift in at least aportion of a threshold voltage distribution (e.g., a shift in magnitudeof voltage stored by at least some of the memory cells in thedistribution) relative to a magnitude of a sensing voltage (e.g.,sensing voltages 310-1 (D*VDM1N) and 311-1 (D VDM1N)) in the sensewindow 312-1. According to embodiments described in further detail inconnection with the “truth table” shown in FIG. 3B, sensing voltages310-2, 311-2, 310-3, and 311-3 may be stepped through, from a lowest,absolute value, voltage magnitude (for negative or positive polarity) tocompare sensed memory states in each of a pair of multi-level memorycells (MLCs), e.g., 301-1 and 301-2, using the various sensing voltages,310-1, 311-1, . . . , 310-3, and 311-3. As shown in FIG. 3A, a magnitudeof a first sensing voltage (e.g., 310-1 (D*VDM1N)) may be selectablycloser to a first threshold voltage distribution (e.g., 308-1) than to asecond threshold voltage distribution (e.g., 308-2) in the sense window312-1.

Although, memory cell 301-1 and memory cell 301-2 are described andillustrated in detail in a negative polarity, embodiments are not solimited. Memory cell 301-1 and memory cell 301-2 may be similarly readin a positive polarity. In some embodiments, the memory cells may beread from the lower sensing voltage to the highest sensing voltage. Thatis, first sensing voltage may have a lesser, absolute value, voltagemagnitude than the second sensing voltage in the negative or positivepolarity. The positive polarity in memory cells 301-1 and 301-2 maystill be read from the lower threshold voltage distribution to thehighest threshold voltage distribution (that is, from 314-1 to 314-4 and315-1 to 315-4). As such, memory cell 301-1 may be read from the lowerthreshold voltage distribution to the highest threshold voltagedistribution, from 308-1 to 308-4 in the negative polarity and from314-1 to 314-4 in the positive polarity. Similarly, memory cell 301-2may be read from the lower threshold voltage distribution to the highestthreshold voltage distribution, as well, from 309-1 to 309-4 in thenegative polarity and from 315-1 to 315-4 in the positive polarity.

As presented in the example of FIG. 3A, the six (6) sensing voltages inthe positive polarity with reference numerals, 316-1, 317-1, 316-2,317-2, 316-3, and 317-3 may be used in three (3) distinct sensingwindows, 318-1, 318-2, and 318-3, to determine the memory states of acorresponding pair of multilevel memory cells (e.g., to distinguishbetween memory states as part of a read operation). Determining thereliability and/or accuracy of the data values may depend on an abilityto distinguish, for example, data value state (01), corresponding tothreshold voltage distribution 314-2, for first memory cell 301-1 in thepositive polarity from data value state (00), corresponding to thresholdvoltage distribution 314-1, for first memory cell 301-1 in the positivepolarity and also from data value state (01), corresponding to thresholdvoltage distribution 315-2, for second memory cell 301-2 in the positivepolarity from data value state (00), corresponding to threshold voltagedistribution 315-1, for second memory cell 301-2 in the positivepolarity.

A determination that the one combination of sensed memory states is morereadably reliable may be based on the greater magnitude of theseparation, thereby having a corresponding greater shift to reach thesensing voltage being less probable than a shift resulting from a lesserseparation in the other combination of sensed memory states.Accordingly, as shown in FIG. 3A, sensing voltage 316-1 (D*VDM1P) isused for memory cell 301-1 in the positive polarity and is at a greaterseparation from threshold voltage distribution 314-2, having a datavalue of 01, than the separation of sensing voltage 316-1 (D*VDM1P) fromthreshold voltage distribution 314-1, having a data value of 00. Inaddition, sensing voltage 317-1 (D VDM1P) is used for memory cell 301-2and is at a greater separation from threshold voltage distribution315-1, having a data value of 00, than the separation of sensing voltage317-1 (D VDM1P) is from threshold voltage distribution 315-2, having adata value of 01.

As used herein, being readably reliable is intended to mean the sensedmemory states being more reliable than, for example as shown in FIG. 3A,threshold voltage distribution 314-1, having a data value of 00, usingsensing voltage 316-1 (D*VDM1P) for memory cell 301-1 in the positivepolarity and sensing of threshold voltage distribution 315-2, having adata value of 01, using sensing voltage 317-1 (D VDM1P) for the intendedmemory cell 301-2 in the positive polarity.

The present disclosure further describes the ability, in some instances,to determine which of two memory cells (e.g., memory cells 301-1 or301-2) has switched their memory state due to a shift in at least aportion of a threshold voltage distribution (e.g., a shift in magnitudeof voltage stored by at least some of the memory cells in thedistribution) relative to a magnitude of a sensing voltage in thepositive polarity (e.g., sensing voltages 316-1 (D*VDM1P) and 317-1 (DVDM1P)) in the sense window 318-1. According to embodiments described infurther detail in connection with the “truth table” shown in FIG. 3B,sensing voltages 316-2, 317-2, 316-3, and 317-3 may be stepped through,from a lowest, absolute value, voltage magnitude (for negative orpositive polarity) to compare sensed memory states in each of a pair ofmulti-level memory cells (MLCs), e.g., 301-1 and 301-2, using thevarious sensing voltages, 316-1, 317-1, . . . , 316-3, and 317-3. Asshown in FIG. 3A, a magnitude of a first sensing voltage (e.g., 316-1(D*VDM1P)) may be selectably closer to a first threshold voltagedistribution (e.g., 314-1) than to a second threshold voltagedistribution (e.g., 314-2) in the sense window 318-1.

FIG. 3B illustrates an example of a truth associated with data valuestates of memory cells for two multi-level memory cells (MLCs) beingsensed to determine multiple data values, in accordance with variousembodiments of the present disclosure. FIG. 3B illustrates a truth tableassociated with various memory states of memory cells, such as memorycells 301 illustrated in FIG. 3A.

Reference to the truth table 313 enables comparison of a sensed memorystate of a first memory cell (e.g., memory cell 301-1) to a sensedmemory state of a second memory cell (e.g., memory cell 301-2),determination of a switch in the memory state due to the shift bydetermination that the sensed memory states match (e.g., both memorystates being duplicated) or are not readably reliable, and determinationof which of the two memory cells has switched. Switching from one memorystate to another memory state is intended to mean that the two memorycells originally stored programed memory states and one of the memorycells switched (changed) its memory state, e.g., due to drift in athreshold voltage distribution. As shown by the truth table this may beindicated by the fact that the two read memory states of the two memorycells match (e.g., a data value of 10 in one memory cell and a datavalue of 10 in the other memory cell), or based on an inability toreliably read combined memory states together in the pair of MLCs. Ininstances shown in the truth table 313 of FIG. 3B where it is possibleto discern which of the two MLCs switched, then responsive to thedetermination of which of the two memory cells has switched from the onememory state to the other memory state (e.g., in reference to the truthtable 313) the circuitry is configured to reprogram the memory cell thathas switched back to a correct memory state. The truth table 313 may bereferenced to enable a determination of which of the two memory cellshas switched their memory state, and in response, circuitry may be usedfor reprogramming the memory cell that has switched its data values backto its original data value.

The determination of which of the two memory cells has switched from onememory state to another memory state is based at least in part on onecombination of the two memory states being more readably reliable thananother combination of the two memory states. For example, as shown inrow 333 of the truth table of FIG. 3B, sensing the memory state 308-1,having a data value of 11 in the first memory cell 301-1 using the firstsensing voltage 310-1 (D*VDM1N), and sensing the memory state 309-2,having a data value of 10 in the second memory cell 301-2 using thesecond sensing voltage 311-1 (D VDM1N) is more readably reliable (e.g.,expressed as resultant valid state “3”). The reliability is due to amagnitude of separation of sensing voltage 310-1 (D*VDM1N) from thethreshold voltage distribution 308-2 of memory cell 301-1 being greaterthan a separation of sensing voltage 310-1 (D*VDM1N) from the thresholdvoltage distribution 308-1 of memory cell 301-1. The reliability isfurther due to a magnitude of separation of sensing voltage 311-1 (DVDM1N) from the threshold voltage distribution 309-1 of memory cell301-2 being greater than a separation of sensing voltage 311-1 (D VDM1N)from the threshold voltage distribution 309-2 of memory cell 301-2. Itis noted that serially stepping through the sensing voltages 310-1(D*VDM1N), 311-1 (D VDM1N), 310-2 (D*VDM2N), 311-2 (D VDM2N), 310-3(D*VDM3N), and 311-3 (D VDM3N) it is only upon application of the fourthsensing voltage 311-2 (D VDM2N) that memory state 309-2, having a datavalue 10 in the second memory cell 301-2 will be detected (e.g., read).The process described herein is carried forward for each possible memorystate combination with the results expressed in the truth table 313 ofFIG. 3B. Similarly, if the sensing voltages applied to each memory cell301-1 and 301-2 sense a result of data value 11 in the second memorycell 301-2 and a result of a data value 10 in the first memory cell(e.g., the inverse to the result shown in row 333) as indicated inreference to row 334 in the truth table 313, then this resultant state(e.g., valid state “4”) may also be readably reliable. Additionally, asshown in row 339, if the results of the applied sensing voltages, 310-1,311-1, 310-2, 311-2 result in a determination that memory cell 301-1 isin a memory state reflected by data value 11 and that memory cell 301-2is also in a memory state reflected by data value 11 then it isdetermined that the memory cell 301-2 has switched state. And as afurther example, as shown in row 338, if the results of the appliedsensing voltages, 310-1, 311-1, 310-2, 311-2 result in a determinationthat memory cell 301-1 is in a memory state reflected by data value 10and that memory cell 301-2 is also in a memory state reflected by datavalue 10 then it is determined that the memory cell 301-1 has switchedstate.

A switch in a memory state of a given memory cell may, in the firstexample above, be due to a shift in a second threshold voltagedistribution (e.g., 309-2) relative to a magnitude of the second sensingvoltage (e.g., 311-1 (D VDM1N)) in the sense window 312-1 rather than tothe magnitude of the first sensing voltage (e.g., 310-1 (D*VDM1N))because of the lesser separation from the second sensing voltage (e.g.,311-1 (D VDM1N)) relative to the first sensing voltage (e.g., 310-1(D*VDM1N)). Because there is a low probability of two paired memorycells switching memory states due to a shift at the same point in time,determining which of the two memory cells has switched their memorystate at that point in time may enable prompt correction of the switchedmemory state back to the original memory state (e.g., before the othermemory cell potentially switches memory states) to improve reliabilityand/or accuracy of stored data.

As used herein, a lesser reliability is intended to mean that anothercombination of the four data values between the two memory cells areless readably reliable due to a first magnitude of separation of asensing voltage from a given threshold voltage distribution representinga memory state and a second magnitude of separation of a second sensingvoltage from another threshold voltage distribution representing anotherbeing less than separations of sensing voltages from the respectivedistributions other memory states in more readably reliablecombinations, or that it is not discernable that both threshold voltagedistributions have not both shifted.

There is a low probability that the same factor would affect thereliability of the two memory states for the two memory cells at thesame time, which contributes to a low probability of two paired memorycells switching memory states at the same point in time. For example, adrift of the first threshold voltage distribution 308-1 to a firstlarger, absolute value, median magnitude may result in at least onememory cell overlapping the closer first sensing voltage 310-1 (D*VDM1N)because the drift would be toward the closer first sensing voltage 310-1(D*VDM1N). In contrast, a drift of the second threshold voltagedistribution 308-2 to a second larger, absolute value, median magnitudewould not result in any memory cells overlapping the closer secondsensing voltage 311-2 (D VDM2N) because the required drift would begreater and the second threshold voltage distribution 308-2 is fartheraway from the fourth sensing voltage 311-2 (D VDM2N). However, as noted,as this process is carried out across all sensing voltages 310-1(D*VDM1N), 311-1 (D VDM1N), 310-2 (D*VDM2N), 311-2 (D VDM2N), 310-3(D*VDM3N), and 311-3 (D VDM3N) for all memory states 308-1, 308-2,308-3, and 308-4 in memory cell 301-1 and memory states 309-1, 309-2,309-3, and 309-4 in memory cell 301-2 it may not always be discernablethat both threshold voltage distributions have not both shifted.

Various factors may contribute to a switch of a memory state due to ashift in at least a portion of a threshold voltage distribution. Forexample, the shift may be contributed to by at least one of: a wideningof the first threshold voltage distribution (e.g., 308-1, 309-1) for afirst subset of memory cells to at least overlap the first sensingvoltage (e.g., 310-1 (D*VDM1N)); a widening of the second thresholdvoltage distribution (e.g., 308-2, 309-2) for a second subset of memorycells to at least overlap the third sensing voltage (e.g., 310-2(D*VDM2N)). The same may occur for each of the threshold voltagedistributions 308-1, 308-2, 308-3, and 308-4 in memory cell 301-1 andthreshold voltage distributions 309-1, 309-2, 309-3, and 309-4 in memorycell 301-2 relative to the other sensing voltages 311-2 (D VDM2N), 310-3(D*VDM3N), and 311-3 (D VDM3N). Shift may also be contributed to by adrift of the threshold voltage distributions in the memory cells to alarger median magnitude of the polarity to at least overlap the sensingvoltages; a disturbance of the memory states of the memory cells byperformance of read or write operations on at least some of the memorycells such that a given threshold voltage distribution at least overlapsa sensing voltage; among other possible contributors to shifting athreshold voltage distribution.

The determination of which of the two memory cells has switched isfurther based at least in part on the circuitry being configured to usethe information depicted the truth table 313, responsive to thedetermination that the two data values corresponding to twocorresponding memory states match or are less readably reliable, and touse the more readably reliable combination of the two memory states fora first memory cell 301-1 and a second memory cell 301-2 of the twomemory cells as a stable combination based on being less probable toswitch. The readably reliability of the two data values are validatedbased on a determination that no SWITCH is likely to occur, as indicatedin the truth table 313 by N/A.

The truth table 313 may indicate one cell or the other as the lessreadably reliable member of the data value pair for the first memorycell 301-1 (“D*”) and the second memory cell 301-2 (“D”). For example, aparticular combination may provide insight as to which cell has switchedbased on being more probable to switch. Because the unreliableness ofthe combination of the data values, it may be determined that the datamay have switched memory states in either the first or the second memorycell 301-1 or 301-2, as indicated in the truth table 313 by the sensingvoltage (D or D*) closest to the threshold voltage distribution. Thetruth table 313 may be further used to compare the valid data values(“valid states”) sensed from the first memory cell 301-1 (D*) and thesecond memory cell (D) to the less readably reliable data values read ofthe two memory states for the first and second memory cells, 301-1 (D*)and 301-2 (D) to determine which of the first or the second memory cells301-1 (D*) and 301-2 (D) has switched.

As shown in the truth table 313, applying the sensing voltages, 310-1(D*VDM1N), 311-1 (D VDM1N), 310-2 (D*VDM2N), 311-2 (D VDM2N), 310-3(D*VDM3N), and 311-3 (D VDM3N) in sequence from lowest absolute voltageto highest absolute voltage to the first memory cell 301-1 (D* in column2 of the truth table 313) and the second memory cell 301-2 (D in column1 of the truth table 313) will result in at least six (6) reliablestates (“valid states”) readable between the two MLCs. These arerepresented as states “1”, “2”, “3”, “4”, “5”, and “6” in the thirdcolumn of the truth table 313 (in the column labeled “Result”), and theat least six (6) reliable states, e.g., valid states, are reflected withthe designation “N.A.” in the fourth column of the truth table 313 for“not applicable” in the column labeled “Fail”.

As a beginning example of operation using the sensing methodologydescribed herein, as the sensing voltages 310-1 (D*VDM1N), 311-1 (DVDM1N), 310-2 (D*VDM2N), 311-2 (D VDM2N), 310-3 (D*VDM3N), and 311-3 (DVDM3N) are stepped through right to left in reference to the negativepolarity from the zero (0) Volt vertical axis in respective absolutemagnitude the following can be determined. As the sensing voltages 310and 311 are applied to the first (D*) and second (D) memory cell 301-1and 301-2, a sensed state having a data value of 10, for the secondmemory cell 301-2 (to which D VDM1N, D VDM2N, and D VDM3N sensingvoltages are applied) and a sensed state having a data value of 11, forthe first memory cell 301-1 (to which D*VDM1N, D*VDM2N, and D* VDM3Nsensing voltages are applied) is a more readably reliable combination(e.g., based on being stable and less probable to switch), as shown inrow 333 in the truth table 313 (e.g., as valid state “3” with an N.A. inthe fail column). An inverse pair of sensed states having a data valueof 11, for the second memory cell 301-2 and a sensed state having a datavalue of 10, for the first memory cell 301-1 is also a more readablyreliable combination, as shown in row 334 in the truth table 313 (e.g.,as valid state “4” with an N.A. in the fail column).

As shown within the truth table 313, an example of a less readablyreliable combination of sensed (e.g., read) states may include thecombination of sensed data values producing a data value of 11, formemory cell 301-2 and a sensed data value of 11, for the first memorycell 301-1. Based on the described technique of placing the sensingvoltages 310-1 (D*VDM1N), 311-1 (D VDM1N), 310-2 (D* VDM2N), 311-2 (DVDM2N), 310-3 (D*VDM3N), and 311-3 (D VDM3N) at opposing ends of therespective sensing windows 312-1, 312-2, and 312-3, this resultant matchof data value states is a less readably reliable combination (e.g.,based on the likelihood of the second memory cell 301-2 (D) to haveswitched), as shown in row 339 in the truth table 313. This is based onreading the reliable state as state 11 (state “3” of the third column)of the first memory cell 301-1 (D*) for the proximity of the sensingvoltage 310-1 (D*VDM1N) in sensing window 312-1 being closer to thefirst threshold voltage distribution of 308-1 in memory cell 301-1. As aresult, the second memory cell 301-2 (D) is reflected in the fourthcolumn (under the “Fail”) as the cell that has switched state (e.g.,“flipped”) and can be reprogrammed back to a correct programmed state inthreshold voltage distribution 309-2 having the data value 10. Forsimilar reasoning, sensed data values producing a data value of 11, formemory cell 301-2 and a sensed data value of 01, for memory cell 301-1is also a less readably reliable combination, as shown in row 340 in thetruth table 313.

Another less reliable sensed data values combination, between memorycell 301-1 and memory cell 301-2, is shown in row 344 in the truth table313. This example, includes the sensed data values of a data value of 01for the second memory cell 301-2 and a sensed data value of 11 for firstmemory cell 301-1. This combination too, based on the sensing voltagemethodology described herein, and is a less readably reliablecombination, expressed as state “6” as shown in row 344 in the truthtable 313. This is based on reading the reliable state as state 11(state “6” of the third column) of the first memory cell 301-1 (D*) forthe proximity of the sensing voltage 310-1 (D*VDM1N) in sensing window312-1 being closer to the first threshold voltage distribution of 308-1in memory cell 301-1. As a result, the second memory cell 301-2 (D) isreflected in the fourth column (under the “Fail”) as the cell that hasswitched state (e.g., “flipped”) and can be reprogrammed back to acorrect programmed state in threshold voltage distribution 309-4 havingthe data value 00 since the sensing voltage D VDM3N is in closerproximity on the end of sensing window 312-3.

Continuing with the example expressed in the truth table 313 of FIG. 3B,applying the sensing voltages 310-1 (D*VDM1N), 311-1 (D VDM1N), 310-2(D*VDM2N), 311-2 (D VDM2N), 310-3 (D*VDM3N), and 311-3 (D VDM3N) atopposing ends of the respective sensing windows 312-1, 312-2, and 312-3to a pair of MLCs 301-1 and 301-2 under this methodology, and readingsensed states having a data value of 00 for memory cell 301-2 and havinga data value of 11 for memory cell 301-1 is determined to be a morereadably reliable combination, as shown in row 346 (as state “6”) in thetruth table 313 (N.A. listed under the fourth, “Fail” columnrepresenting a valid state). Again, the proximity of sensing voltage310-1 (D*VDM1N) to threshold voltage distribution 308-1 for memory cell301-1 in sensing window 312-1 and the proximity of sensing voltage 311-3(D VDM3N) to threshold voltage distribution 309-4 for memory cell 301-2in sensing window 312-3 produces the high likelihood of a valid stateread. Similarly, sensed data values according to this disclosedmethodology having a data value of 11 for memory cell 301-2 and having adata value of 00 for memory cell 301-1 is also a more readably reliablecombination, as shown in row 341 (state “5”) in the truth table 313(N.A. listed under the fourth, “Fail” column representing a validstate).

As shown within the truth table 313 in row 338, another example of aless readably reliable combination of sensed (e.g., read) states mayinclude the sensed data values of 10 for the second memory cell 301-2,and a sensed data value of 10 for first memory cell 301-1. Thiscombination too, based on the sensing voltage methodology describedherein, and is a less readably reliable combination, expressed as state“3” as shown in row 338 in the truth table 313. This is based on readingthe reliable state as state 10 (state “3” of the third column) of thesecond memory cell 301-2 (D) for the proximity of the sensing voltage311-1 (D VDM1N) in sensing window 312-1 being closer to the secondthreshold voltage distribution of 309-2 in memory cell 301-2. As aresult, the first memory cell 301-1 (D*) is reflected in the fourthcolumn (under the “Fail”) as the cell that has switched state (e.g.,“flipped”) and can be reprogrammed back to a correct programmed state inthreshold voltage distribution 308-1 having the data value 11 since thesensing voltage D*VDM1N is in closer proximity on the end of sensingwindow 312-1.

There may be sensed data combinations that may be unverified. Forexample, the combination of sensed data value of 01, for memory cell301-2 and send data value of 10, for memory cell 301-1 may be anunverified combination, as shown in row 343 in the truth table 313. Theproximity of sensing voltage 310-2 (D*VDM2N) to threshold voltagedistribution 308-2 for memory cell 301-1 in sensing window 312-2 and theproximity of sensing voltage 311-2 (D VDM2N) to threshold voltagedistribution 309-3 for memory cell 301-2 in sensing window 312-2produces the high likelihood of a valid state read. However thiscombination does not produce a reliable combination of sensed datavalues and it is uncertain which cell (301-1 or 301-2) switched states.As such, the third column is labeled as N.V. (“not valid”) and thefourth/“fail column” is left blank. Likewise the combination of senseddata value 10, for memory cell 301-2 and sensed data value of 01, formemory cell 301-1 may also be a may be unverified combination, as shownin row 342 in the truth table 313. The proximity of sensing voltage310-2 (D*VDM2N) to threshold voltage distribution 308-3 for memory cell301-1 in sensing window 312-2 and the proximity of sensing voltage 311-2(D VDM2N) to threshold voltage distribution 309-2 for memory cell 301-2in sensing window 312-2 produces the high likelihood of an “invalid”state read for either of the MLCs 301-1 and 301-2 being sensed. Thus,this combination does not produce a reliable combination of sensed datavalues and it is uncertain which cell 301-1 or 301-2 switched states. Assuch, the third column is labeled as N.V. (“not valid”) and thefourth/“fail column” is left blank.

Pairing the combination having sensed data values of 00, for memory cell301-2 and sensed data values of 10, for memory cell 301-1 is a lessreadably reliable combination, as shown in row 345 in the truth table313. This is based on reading the reliable state as state 00 (state “6”of the third column) of the second memory cell 301-2 (D) for theproximity of the sensing voltage 311-3 (D VDM3N) in sensing window 312-3being closer to the fourth threshold voltage distribution of 309-4 inmemory cell 301-2. As a result, the first memory cell 301-1 (D*) isreflected in the fourth column (under the “Fail”) of row 345 as the cellthat has switched state (e.g., “flipped”) and can be reprogrammed backto a correct programmed state in threshold voltage distribution 308-3having the data value 01. An inverse pair of sensed states having a datavalue of 10, for memory cell 301-2 and threshold voltage distribution308-1, having a data value of 00, for memory cell 301-1 is also a lessreadably reliable combination, as shown in row 337 in the truth table313. This combination too, based on the sensing voltage methodologydescribed herein, is a less readably reliable combination, expressed asstate “2” as shown in row 337 in the truth table 313. This is based onreading the reliable state as state 00 (state “2” of the third column)of the first memory cell 301-1 (D*) for the proximity of the sensingvoltage 310-3 (D*VDM3N) in sensing window 312-3 being closer to thefourth threshold voltage distribution of 308-4 in memory cell 301-1. Asa result, the second memory cell 301-2 (D) is reflected in the fourthcolumn (under the “Fail”) as the cell that has switched state (e.g.,“flipped”) and can be reprogrammed back to a correct programmed state inthreshold voltage distribution 309-3 having the data value 01 since thesensing voltage D VDM3N is in closer proximity on the end of sensingwindow 312-3.

Another example of a less readably reliable may include the combinationof sensed data value of 01, for memory cell 301-2 and sensed data valueof 01, for memory cell 301-1 is a less readably reliable combination, asshown in row 335 in the truth table 313. This combination, based on thesensing voltage methodology described herein is a less readably reliablecombination, expressed as state “1” as shown in row 335 in the truthtable 313. This is based on reading the reliable state as state 01(state “1” of the third column) of the first memory cell 301-1 (D*) dueto the proximity of the sensing voltage 310-3 (D*VDM3N) in sensingwindow 312-3 being closer to the fourth threshold voltage distributionof 308-3 in memory cell 301-1. As a result, the second memory cell 301-2(D) is reflected in the fourth column (under the “Fail”) as the cellthat has switched state (e.g., “flipped”) and can be reprogrammed backto a correct programmed state in threshold voltage distribution 309-4having the data value 00 since the sensing voltage D VDM3N is in closerproximity on the end of sensing window 312-3.

Pairing the combination of sensed data value 00, for memory cell 301-2and sensed data value 01, for memory cell 301-1 is a more readablyreliable combination, as shown in row 331 in the truth table 313. Theproximity of sensing voltage 310-3 (D*VDM1N) to threshold voltagedistribution 308-3 for memory cell 301-1 in sensing window 312-3 and theproximity of sensing voltage 311-3 (D VDM3N) to threshold voltagedistribution 309-4 for memory cell 301-2 in sensing window 312-3produces the high likelihood of a valid state read. Similarly, senseddata values according to this disclosed methodology having a data valueof 01 for memory cell 301-2 and having a data value of 00 for memorycell 301-1 is also a more readably reliable combination, as shown in row331 (state “2”) in the truth table 313 (N.A. listed under the fourth,“Fail” column representing a valid state).

Pairing the combination of sensed data value 00, for memory cell 301-2and sensed data value 00, for memory cell 301-1 is a less readablyreliable combination, as shown in row 336 in the truth table 313. Thiscombination as well, based on the sensing voltage methodology describedherein is a less readably reliable combination, expressed as state “1”as shown in row 336 in the truth table 313. This is based on reading thereliable state as state 00 (state “1” of the third column) of the secondmemory cell 301-2 (D) for the proximity of the sensing voltage 311-3 (DVDM3N) in sensing window 312-3 being closer to the fourth thresholdvoltage distribution of 309-4 in memory cell 301-2. As a result, thefirst memory cell 301-1 (D*) is reflected in the fourth column (underthe “Fail”) of row 336 as the cell that has switched state (e.g.,“flipped”) and can be reprogrammed back to a correct programmed state inthreshold voltage distribution 308-3 having the data value 01.

As such, the truth table 313 may be used to enable determining which ofthe two memory cells has switched their memory state due to a shift inits threshold voltage distribution relative to a magnitude of acorresponding sensing voltage. The determination may be based oncomparing a pair of data values determined to be reliable for the firstand second memory cells to a sensed second data value pair of the firstand second memory cells. The determination may be further based ondetermining which of the two memory cells has switched their memorystate based on which memory cell has a sensed memory state that differsfrom the first data value pair for the first and second memory cells.

Although, truth table 313 is described based on a negative polarity,embodiments are not so limited. Truth table 313 may be similarly read ina positive polarity to that described above in association with thenegative polarity operation.

FIG. 4 is a block diagram illustration of an example apparatus, such asan electronic memory system 400, in accordance with an embodiment of thepresent disclosure. Memory system 400 may include an apparatus, such asa memory device 402 and a controller 404, such as a memory controller(e.g., a host controller). Controller 404 might include a processor, forexample. Controller 404 might be coupled to a host, for example, and mayreceive command signals (or commands), address signals (or addresses),and data signals (or data) from the host and may output data to thehost.

Memory device 402 includes a memory array 406 of memory cells. Forexample, memory array 406 may include one or more of the memory arrays,such as a cross-point array, of memory cells disclosed herein. Memorydevice 402 may include address circuitry 408 to latch address signalsprovided over I/O connections 410 through I/O circuitry 412. Addresssignals may be received and decoded by a row decoder 414 and a columndecoder 416 to access the memory array 406. For example, row decoder 414and/or column decoder 416 may include drivers.

Memory device 402 may sense (e.g., read) data in memory array 406 bysensing voltage and/or current changes in the memory array columns usingsense/buffer circuitry that in some examples may be read/latch circuitry420. Read/latch circuitry 420 may read and latch data from the memoryarray 406. Sensing circuitry 405 may include a number of senseamplifiers coupled to memory cells of memory array 406, which mayoperate in combination with the read/latch circuitry 420 to sense (read)memory states from targeted memory cells. I/O circuitry 412 may beincluded for bi-directional data communication over the I/O connections410 with controller 404. Write circuitry 422 may be included to writedata to memory array 406.

Control circuitry 424 may decode signals provided by control connections426 from controller 404. These signals may include chip signals, writeenable signals, and address latch signals that are used to control theoperations on memory array 406, including data read and data writeoperations.

Control circuitry 424 may be included in controller 404, for example.Controller 404 may include other circuitry, firmware, software, or thelike, whether alone or in combination. Controller 404 may be an externalcontroller (e.g., in a separate die from the memory array 406, whetherwholly or in part) or an internal controller (e.g., included in a samedie as the memory array 406). For example, an internal controller mightbe a state machine or a memory sequencer.

In some examples, controller 404 may be configured to cause memorydevice 402 to at least perform the methods disclosed herein, such as twomulti-level memory cells being sensed to determine multiple data values.In some examples, memory device 402 may include the circuitry previouslydescribed in conjunction with FIGS. 1, 2A-B, and 3A-B. For example,memory device 402 may include circuitry such as the sense amplifiers,the latches, the truth tables, the word and bit lines, and/or the pairedmemory cells, among other circuitry disclosed herein.

As used herein, the term “coupled” may include electrically coupled,directly coupled, and/or directly connected with no intervening elements(e.g., by direct physical contact) or indirectly coupled and/orconnected with intervening elements. The term coupled may furtherinclude two or more elements that co-operate or interact with each other(e.g., as in a cause and effect relationship).

It will be appreciated by those skilled in the art that additionalcircuitry and signals can be provided, and that the memory system 400 ofFIG. 4 has been simplified. It should be recognized that thefunctionality of the various block components described with referenceto FIG. 4 may not necessarily be segregated to distinct components orcomponent portions of an integrated circuit device. For example, asingle component or component portion of an integrated circuit devicecould be adapted to perform the functionality of more than one blockcomponent of FIG. 4. Alternatively, one or more components or componentportions of an integrated circuit device could be combined to performthe functionality of a single block component of FIG. 4.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of a number of embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of ordinary skill in the artupon reviewing the above description. The scope of a number ofembodiments of the present disclosure includes other applications inwhich the above structures and methods are used. Therefore, the scope ofa number of embodiments of the present disclosure should be determinedwith reference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

What is claimed is:
 1. An apparatus, comprising: a memory having aplurality of memory cells; and circuitry configured to sense memorystates of each of two self-selecting multi-level memory cells (MLC) ofthe plurality of memory cells to determine multiple data values by:sensing a memory state of a first MLC using a first sensing voltage in asense window between a first threshold voltage distributioncorresponding to a first memory state and a second threshold voltagedistribution corresponding to a second memory state; sensing a memorystate of a second MLC using a second sensing voltage in a sense windowbetween a first threshold voltage distribution corresponding to a firstmemory state and a second threshold voltage distribution correspondingto a second memory state; sensing the memory state of the first and thesecond MLCs using sensing voltages higher, in absolute magnitude values,than the first and the second sensing voltages in subsequent sensingwindows, in repeated iterations, until the state of the first and thesecond MLCs are determined; and wherein the first sensing voltage andthe second sensing voltage are selectably closer in the sense window tothe first threshold voltage distribution or the second threshold voltagedistribution.
 2. The apparatus of claim 1, wherein the circuitrydetermines data values by: sensing a memory state of the first MLC usinga third sensing voltage in a sense window between the second thresholdvoltage distribution corresponding to the second memory state and athird threshold voltage distribution corresponding to a third memorystate; sensing the memory state of the second MLC using a fourth sensingvoltage in the sense window between the second threshold voltagedistribution corresponding to the second memory state and a thirdthreshold voltage distribution corresponding to the third memory state;and wherein the third sensing voltage and the fourth sensing voltage areselectably closer in the sense window to the second threshold voltagedistribution or the third threshold voltage distribution.
 3. Theapparatus of claim 1, wherein the circuitry determines data values by:sensing a memory state of the first MLC using a fifth sensing voltage ina sense window between the third threshold voltage distributioncorresponding to the third memory state and a fourth threshold voltagedistribution corresponding to a fourth memory state; sensing a memorystate of the second MLC using a sixth sensing voltage in the sensewindow between the third threshold voltage distribution corresponding tothe third memory state and a fourth threshold voltage distributioncorresponding to the fourth memory state; and wherein the fifth sensingvoltage and the sixth sensing voltage are selectably closer in the sensewindow to the third threshold voltage distribution or the fourththreshold voltage distribution.
 4. The apparatus of claim 1, wherein thefirst and second MLCs each comprise 2 bits and provide a combination of4 memory states.
 5. The apparatus of claim 1, wherein the first sensingvoltage is lower than the second sensing voltage.
 6. The apparatus ofclaim 3, wherein a sensing voltage determines a reliability of athreshold voltage distribution.
 7. An apparatus, comprising: circuitryin a memory device, the circuitry configured to: sense memory states ofeach of two self-selecting multi-level memory cells (MLCs) of aplurality of memory cells to determine multiple data values by: sensinga memory state of a first MLC using a first sensing voltage in a sensewindow between a first threshold voltage distribution corresponding to afirst memory state and a second threshold voltage distributioncorresponding to a second memory state; sensing a memory state of asecond MLC using a second sensing voltage in a sense window between thefirst threshold voltage distribution corresponding to a first memorystate and a second threshold voltage distribution corresponding to thesecond memory state; sensing the memory state of the first and thesecond MLCs using higher sensing voltages than the first and the secondsensing voltages in subsequent sensing windows, in repeated iterations,until the state of the first and the second MLCs are determined; anddetermining which of the two memory cells has switched their memorystates due to a shift in the first threshold voltage distributionrelative to a magnitude of the first sensing voltage in the sensewindow; wherein the magnitude of the first sensing voltage is selectablycloser to the first threshold voltage distribution than to the secondthreshold voltage distribution in the sense window.
 8. The apparatus ofclaim 7, wherein each self-selecting memory cell comprise a singlechalcogenide material which serves as a select element and a storageelement.
 9. The apparatus of claim 7, wherein the combination of a firstmemory cell in the MLC located at a highest voltage of the negativepolarity and a second memory cell in the MLC located at a lowest voltageof the positive polarity do not switch memory states.
 10. The apparatusof claim 7, wherein the value of the memory state is determined beforeswitching from the first threshold voltage to a second thresholdvoltage.
 11. The apparatus of claim 7, wherein the first sensing voltageis closer to the first threshold voltage distribution and the secondsensing voltage is closer to the second threshold voltage distributionin the sense window.
 12. The apparatus of claim 7, wherein a firstsensing voltage and the second sensing voltage are selectably closer inthe sense window to different ones of the first threshold voltagedistribution and the second sensing voltage.
 13. The apparatus of claim7, wherein a position of the first sensing voltage and the secondsensing voltage are fixed.
 14. The apparatus of claim 7, wherein theMLCs are programmed in a negative polarity for a negative read using oneof a pair of complementary memory states for the first memory cell and apositive polarity for a positive read using another of the pair for thesecond memory cell.
 15. The apparatus of claim 7, wherein the circuitryis further configured to program the data values by: applying a firstvoltage pulse to the first memory cell, wherein the first memory cell isdesignated to store its data values; and applying a second voltage pulseto the second memory cell, wherein the second memory cell is designatedto store its data values as a comparison for the first memory cell datavalues to find which memory cell switched its memory state.
 16. Theapparatus of claim 7, wherein each of the two memory cells is aresistance variable memory cell configured to selectably store either ofthe first and second states.
 17. A method of operating memory,comprising: sensing memory states of two multi-level memory cells (MLCs)of a plurality of memory cells to determine four data values and sixteencombinations by: sensing a memory state of a first MLC using a firstsensing voltage in a sense window between a first threshold voltagedistribution corresponding to a first memory state and a secondthreshold voltage distribution corresponding to a second memory state;sensing a memory state of a second MLC using a second sensing voltage ina sense window between the first threshold voltage distributioncorresponding to a first memory state and a second threshold voltagedistribution corresponding to the second memory state; sensing thememory state of the first and the second MLCs using higher sensingvoltages than the first and the second sensing voltages in subsequentsensing windows, in repeated iterations, until the state of the firstand the second MLCs are determined; and selectably positioning the firstsensing voltage and the second sensing voltage closer in the sensewindow to the first threshold voltage distribution or the secondthreshold voltage distribution.
 18. The method of claim 17, furthercomprising: comparing the sensed memory states of each of the two memorycells; validating the memory state of the first memory cell as anintended data value corresponding to the four data values by determiningthat the sensed memory states are complementary binary memory states;and determining a switch in the memory state of the two MLCs bydetermining that the sensed memory states match binary memory states.19. The method of claim 17, further comprising reading the data valueswithin the two MLCs as a unit.
 20. The method of claim 17, furthercomprising switching memory states due to a shift in the secondthreshold voltage distribution relative to a magnitude of the secondsensing voltage in the sense window.